readline: add missing documentation, fix documentation format, add README.md (#7683)

vlib/readline/README.md

@@ -0,0 +1,20 @@
+# Readline
+
+The `readline` module let you await and read user input
+from a terminal in an easy and structured manner.
+
+The module provides an easy way to prompt the user for
+questions or even make a REPL or an embedded console.
+
+Use `readline.Readline` if you want to include more
+advanced features such as history or simply use
+`readline.read_line('Please confirm (y/n):')` directly
+for one-off user interactions.
+
+# Usage
+
+```v ignore
+import readline { Readline }
+
+Readline.read_line('Continue?: (y/n)')
+```